reserve A for QC-alphabet;
reserve p, q, r, s, p1, q1 for Element of CQC-WFF(A),
  X, Y, Z, X1, X2 for Subset of CQC-WFF(A),
  h for QC-formula of A,
  x, y for bound_QC-variable of A,
  n for Element of NAT;

theorem Th64:
  p = h.x & x <> y & not y in still_not-bound_in h implies not y
  in still_not-bound_in p
proof
  assume that
A1: p = h.x and
A2: x <> y and
A3: not y in still_not-bound_in h and
A4: y in still_not-bound_in p;
A5: still_not-bound_in p c= still_not-bound_in h \/ {x} by A1,Th63;
  per cases by A4,A5,XBOOLE_0:def 3;
  suppose
    y in still_not-bound_in h;
    hence contradiction by A3;
  end;
  suppose
    y in {x};
    hence contradiction by A2,TARSKI:def 1;
  end;
end;
